Process for the esterification of cellulose



Aug; 2s, 1941.

A. A. NEW E11-AL PROCESS FOR THE ESTERIFICATION OF CELLULOSE Filed Jan. 20, 1940 d l mn z ozhwumhwf 9u mx z .w 2 15.5.5 952.. .3G50 .dwwm mr 22a o n. m v. www z m m m m Q n /l n n v Q O U.

Patented Aug. 26, 1.941

PATENT FFICi?.l

PROCESS FOR THE 'ESTERIFICATION'OF CELLULOSE Archibald Alan New, Dudley Robert Beckwith,

and William Ashley Wiltshire, London, England, assignors to International Standard Electric Corporation, New York, N. Y.

Application .mmm zo, 1940, serial No. 314,110 In Great Britain Jannaryls. 1939 Claims.

This invention relates to a process for the esteriilcation of cellulose to form esters of carboxylic acids having from 2-5 carbon atoms with retention of the fibrous form.

Such processes are known -and have been carried out by means of a controlled esterification, that is, the process'necessary for converting the cellulose into aA tri-ester in solution has been carried out at aslower rate and in the presence of a' non-solvent for vthe tri-ester, the process of esterication taking a period of some hours, even after previous soaking of the libres for some hours in the acid, the ester of which is to be produced, and a catalyst. A comparatively mild catalyst such as zinc chloride or a salt of perchloric acid has been used. l

It has also been found previoushr (British Patent No. 494,253) that, on carrying out a rapid esterication process of paper to form an ester of "a carboxylic acid containing from 2 to 5 carbon atoms at a temperature higher than 60C., whilst preserving the brous structure, the ilrst result was an esteriilcation of the 'surface layers only of the fibres, so that by carrying out the process for atleast 5 minutes and for periods up to at least 20 minutes durationv without pre-treatment with the acid, the tensile strength when wet wasV equal to that of paper having a much greater combined carboxylic acid content that had been prepared 'by the usual processes for uniform esteriiication. It is to be noted that esteriiied paper prepared according to this last mentioned process was not *in general suitable for electric insulation purposes because of its non-uniform esteriilcation.

The present'invention comprises a process for the substantially uniform esteriilcation of cellulose to forin an ester or esters of acetic, propionic or butyric acids with retention of the fibrous form (Cl. E60-227) 5 or complete, can be carried out in periods that are extraordinarily brief as compared with the prior art and the fibrous structure be retained.

As an `example'of a process'according to the ,invention a sheet of 5 mil wood-pulp manila (40-60) paper (undried) is passed through a bath of glacial acetic acid to which has been added l5% by weight of commercial perchloric acid (sp. gr. 1.695) during a period of 33 seconds at room temperature and then through a bath containing l5 70% acetic anhydride and 30% toluene at a temperature of 49 C. for a period of 37 seconds.

The sheet of paper then passes through the air at room temperature for 78 seconds and then into a washing bath of water and is afterwards dried.

The paper then has a combined acetic acidl content of 30%.

Paper esteriiied in accordance with the above example is found to be equal in-electrical properties to paper esteritled in accordani'u'e with knownV vuniform esteriiication processes.

The exact conditions required to esterify to a .given degree depend upon the particular kind o f paper. For example, a denser paper requires l, slower running through the plant than a less dense paper to reach the same degree of esterilication. A thinner paper esterifles more quickly than a thicker paper of the same grade, up to a certain degree of esteriiication.

The combined acetic acid content can be controlled, up to full acetylation, by the temperature of the anhydride bath. 'The following table shows the relation between the temperature of the anhydride bath in degrees centigrade and the combined acetic content at the conclusion of the .in which perchloric acid is used as a catalyst and, 40 process given in 'the example above, carried out the cellulose is passed in the form oi'a continuous sheet or thread through a bath or baths in -which the esterlilcation. takes place. This has upon diil'erent thicknesses of paper of the same grade, i. e. a manila 40% wood pulp cable Table 1 1% mil. 2% mil. '3% mil. li mii. 7 mil.

' combined combined combined combined combined Temp. acetic acid Temp. acetic acid Temp. acetic acid Temp. acetic acid Temp acetic acid content content content content content c. Poem c'. man: c. Pmm: c. Paw.: c Pimm 30 18.0 30 15.8 28 11.9 30 0.0 3.8 37.5 10.8 37.5 19.2 '3s .12.4 4o las 38.0 6.0 49 28.4. .51.5 27.0 60.5 22.8 4s 26.0 49.0 21.4 ses 41.2 can 42.5 61.0 .l 43.8 as 40.0- cco 45.2 65.0 54.0 65.0 64.0 .70.0 68.8 70 50.0' 69.5 00.0 l72.5 .aio n.6 can One mil. isone thousandth oi an inch. It will be observed that for lower temperatures thinner papers acetylate to a higher combined acetic acid content than .thicker papers, but that at higher temperatures` the reverse is sometimes v the case. The above figures are generally representative of theresults obtained, but owing to slight. dinerences in diiferent portions of a given paper, the acetylation curves on repetition will not always .follow exactly the .same course.

It willalso he noted that between the temperatures 60-75" the acetylation speed increases [until nearly maximum acetylation is reached (about 58%) after which the speed decreases rapidly.

It is to be emphasized that the process according to the invention can be carried out for all degrees of esteriflcation up to full esteriiication. For many electrical purposes the improvement in insulation resistance under humid conditions obtained by carrying the process up to or beyond the di-acyl stage is not suiliciently great to be economically 'justified but for use as electric insulation at high frequencies, i. e. kc. and above, esteriflcation of paper as well as cotton beyond the diacyl stage is iustied, since it leads to a decrease in the power factor as well as an increase in insulation resistance as compared with the same material unesteried or esterified below the diacylstage.

The moisture content of the original paper has an eifect uponl the combined acid content for a given time and temperatureof reaction. In general, previous drying of the' paper decreases the combined acetic acid content for a given temperature of the anhydride bath and speed of running. The following table gives the combined acetic acid contents obtained `with a process according to the example mentioned above carried out upon a' 21/2 mil all wood cable paper, andv with the sameprocess carried out upon the same paper after it had been dried by being passed over a heated roller. Before drying the moisture content was 7%, after drying less than 1%.

Table 2 Combined acetic acid Temperature of anhydride bath Undi'ied Dried sans; WQINOC are also slightly improved. The following table 75 content, mnt A 20 were obtained by measurement of an area of 50 square centimeters under an applied pressure of 100 grams per sq. cm.

Tabled Transverse direct current insulation resist- Combined acetic acid content n9 megohms Dried Undried The difference between the two sets of iigures4 appears to be due to smoother acetylation taking place when water in the paper has been displaced 40 by acetic acid. On the other hand, the treatmentl of undried paper bythe process described in the above example leads to the weakening of the catalyst liquor. In order to retain the advantages of the treatment of `undried paper without this resultant disadvantage, the paper may be passed through a bath ot glacial acetic acid before passing through the glacial acetic acid plus catalyst bath. In order to retain theefilciency of dehydration it is necessary to take care that the dehydration emciency of the acetic acid bath does not decrease unduly. We have found that below 2%l of moisture in the acetic acid the eiilciency of dehydration is very nearly 100%, and that it is not advisable to allow the dehydration eiliciency to fall below 60%, which corresponds to a moisture content of 6.7% in the acetic acid bath. We have found that 14,500 lbs. of paper can be passed through an acetic acid bath containing 10,000 lbs. of acid before the moisture content of 50 a paper originally containing 6% of moisturefalls In order to avoid too frequent attention to the acetic acid dehydration bath it may be advisable to effect a preliminary drying of the paper by o5 means of heated rollers to reduce its moisture content to 3% 4before passing it through the de hydration bath. The weight of paper that could then be passed through a tank containing 10,000

lbs. of acetic acid before the dehydration fails to be effective'is 29,000 lbs.

The proportions of catalyst to acetic acid may be varied widely with corresponding variations in `the temperatures and/or times of running through the bath for a-given acetic acid content.

Too great a proportion of perchloric acid. how-4 ever, leads to tendering of the bres particularly for higher degrees of acetylation. 'I'hus 10% of perchloric acid leads to tendering of the ilbres for any degree of acetylation. Assumingfor purposes ofl `comparision the times of passage through the baths given in the example previously quoted, 7% of perchloric acid may -be used 11D to a temperature of 60 C. at whichtemperature 32.5% combined aceticacid content results, but at 70 C. (combined acetic acid content 47%) slighttendering takes place. Lower concentra` tions of catalyst than may also be used and over the useful range the resultant combined acetic acid content is approximately a. linear function of the concentration of catalyst. 5% of perchloric acid is the optimum value for the production of acetylated paper 'to a combined acetic acid content of over 10%, but for the production ofvery low acetylated paper, 2% is mosteconomical.

u i in order tol obtain a predetermined combined ace- 'I'he presence of -a diluent such as toluene in the anhydride bath is not essential, and slightly hlgher'combined acetic acid contents at thevarious temperatures are .obtained by the use ofa 100% anhydride bath, as shown by the following -ta'ble of results obtained with a 60% manila 40% wood pulp 5 mil paper;

the electrical properties slightly as-illustrated by the following figures for the transverse direct current insulation resistance of the same paper, taken under the same conditionsl as for Table 3 The speed at which the paper is passed through l the diilerent.tanks may be varied with conse-l quent variation in the degree of esteriiication. A change in this speed will of course alter the other -ccnditions,.fi. e. length of time 4during which the paper is immersedin each bath.

The following table summarises the results obtained by varying the The results shown were obtained in the acetylation of paper with the use of a preliminary bath of glacial acetic acid as described above.

Table 6 Immersion Combined acetic acid content 'time infor temperature oianhydride Running speed `in bathin itJmin. secs. 40 O. 50 C. 60 C. 65 C.V

lt will-be'seen therefore that the process according to the invention is very easilyv controlled or less dense land the properties which are re. .quired from the resultant paper. The fixing of one variable automatically fixesthe other for a combined vacetic acid content. l

The process according to the invention is applicableto the formation of cellulose esters of propionic `and butyrlc acids. VThe combined acid content obtained with a given temperature and speed of passage through the various baths is' low- 1 The presence` of a diluent, however. improves 1 are given in Table 7.

. er the greater the number of carbon atoms in the acid concerned. As an illustration, a sheet of -5 mil 60% manila 40% wood pulp paper is passed through a bathof butyrlc acid to which has been added 5% by weight of perchloric-acid, (sp. gr. 1.695) during a period of 33 seconds at room temperature and then through a bath containing butyrlc anhydride'and 30% toluene foraperiod of 37 seconds. The temperature of this bath was varied from time to time and the portions -o f the sheet passed through it at the different tem ratures after washing and drying tested for combined butyric acid content. In all cases, however, the sheet passed through the air at room temperature for 78 secondsto complete the reaction. e

vThe different combined butyrlc ,acid contents above. Table 7 Table 5 50 bCznbinedd n c Transverse direct curv Temperature C' @Olltellltlalfrrent insulation re- 'nt sistanoe in L Combined acetic acid content, percent 5 30%toluene 10 1.7 v c 107 107 10 ato 1,100 Thus for full esteriflcation in the case of butyrlc 9g 31% 60 acid the time of immersion in the anhydridebath 40 10,000 45,000 at 60 C. would be approximately 140 secs. g gw 1,2% It will be seenthat considerable variation in the details of the process may be made. Thus in addition to variations previously noted, solvents for cellulose tri-acetate, such as acetic acid, may be added to the anhydride bath, with or without the presence of inert diluents such as toluene,

' benzene or .the like. In some circumstances also a single bath containing acid, anhydride and catalyst, with orwithout a 'diluent may be used.

. lIt is to be noted also that the process is applicable to the formation, of mixed esters of acetic, propionic orbutyric acids.

. It should be noted also that, although the invention is -of particular value when applied to paper as compared with known processes, since esterltlcation of paper in roll form may give somewhat uneven results. it is equally applicable to other forms oi cellulose such as cotton in the form of tapes or sheets.

In vthe single figure of the drawing we have shown a schematic arrangement for acetylating paper in a continuous manner. This drawing shows by way of example, a plant for the acetylation oi paper in a continuous roll. The roll o! paper is shown at 4. The paper is drawn from the roll I between drying rollers l, which are con- .veniently heated by passing steam through them in well known manner. The sheet o! paper then passes over and between rollers as shown through tanks A. B and C and thence through a chamber D maintained atatmospheric temperature. The paper then passes through a passage D', which is heated in any convenient manner and through washingtanksE,FandGandontoatakeup roller.

'I'he rollers marked 3 are mounted in such manner :sesam A 'moftnennnaamcissomas rnc combined acetic acid content required is thenl controlled by controlling the temperature o! the tankCaspreviously'desci-ibed. l y

In theinachine shown the paper is .woimd in the wet condition on the nnal roll-I. The paper is then dried and it may be calendered in a separate operation. This has been found preferable since tlie speed oi ev through `the dryingchambercanthen be more readilycontrolled ln'dependentlyoi the speed of acetylation than if acetylation. washing, drying and -calendering f were all perfumed in one continuous operation.

that they can be raised into position for initially l pasing the paper around them at the commencement Ao! operations. The rollers 9 at the en.- -trance to the tank E are heated and arranged after the manner of calendering rollers. Pipes l0 are arranged as shown transversely to the sheet of paper and are perforated and fed with water under pressure so that a spray oi water is directed on the sheet of paper. The tanks A, B and C and chamber D are surmounted by a hood 1 having an exhaust opening 8 above which is mounted an exhaust fan (not shown). The passage D' is continued in a tube il which leads to a plant in which any fumes vare extracted and any evaporated liquor condensed and recovered in well knownmanner.

The heated rollers 3 are heated to such temperature that the moisture content of the paper is reduced to 3%. The tank A contains glacial acetic acid, the tank B contains glacial acetic acid with 5% perchloric acid, and the tank C contains 70% acetic anhydride and 30% toluene. The tank C is tted with pipes (not shown) through which steam or hot water may be circu- /lated and pipes through which cold water may be circulated and thermostatic control means o! any Well known kind is provided to regulate the ilow of heating and/or cooling medium -through these pipes in order to keep the temperature constant at a desired figure. Stirring means (not 4 shown) is also provided in this tank C. The

tanks E, F and G are washing tanks containing Water. The speed at which the take up roll 5 and the driving rollers i are driven is conven- What is claimed is:`

i. Aqulck, continuous process for the esterilication oi cellulose in continuous lengths which comprises the steps ot continuously the cellulose through a bath of anl aliphatic carboxylic acid having from two to ve carbon atoms and containing 2% to 7% voi perchloric acid. and then passing the thus treated cellulose continuously through a bath composed oi an anhydride ci an aliphatic carboxylic acid having from two to live. carbon atoms.

2. A quick continuous process for esteriiying cellulose in lengths which comprises continuously subjecting the cellulose length to the action oi glacial acetic acid containing 2% to 7% oi perchloric acid and then continuously passing the thus treated cellulose length into a bath containing acetic anhydride.

3. A quick continuous process .for ,esteriiying cellulose in lengths which comprises the steps of at least partially removing moisture irom'the cellulose, then continuously passing the cellulose into contact with an aliphatic carboxylic acid containing 2% to 7% of perchloric acid, and ccntinuously passing the thus treated cellulose into contact with an anhydride of an aliphatic car- -boxylic acid having from two to ilve carbon atoms.

4. A quick continuous process for esteriiying cellulose in lengths which comprises the steps oi iently adjusted such that time of immersion in v at least partially removing moisture from the cellulose, and then continuously passing the cellulose into contact with glacial acetic acid containing 2% to '1% oi perchlorlc acid, and then continuously passing the thus treated cellulose into contact with acetic anhydride.

5. In the esterication of cellulose to form esters of carboxylic acids containing from two to ve carbon atoms while retaining the tensile strength and iibrousstructure oi the cellulose the step which comprises subjecting the cellulose vto the action of such acid and of the anhydride of such acid, at least one of such actions beingV in the presence of between 2% and 5% oi' perchloric acid.

ARCHIBALD ALAN NEW. DUDLEY ROBERT BECKWITH. Y WILLIAM Ail-lHI..liY WILTSHIRE. 

